The present invention relates to methods for disinfecting water and other dense fluid media in a dense medium plasma environment.
Decontamination and disinfection of potable water, water used in food-processing industries, and water frequently in contact with human beings (e.g. water in swimming pools and spa pools), are major health issues currently under intense scrutiny due to heightened awareness. Disinfection is defined as the killing or inactivation of disease-causing organisms. The levels to which microbial colony forming units are permitted in various waters fit for human contact is carefully regulated. Conventional approaches employed for the inactivation of toxins, such as hydrolysis, electrochemical oxidation, solvated electron technology, plasma arcs, and chemical treatments are complex processes with significant limitations related to the generation of toxic side-products or low efficiencies for large scale applications.
Technologies based on atmospheric pressure plasma environments present an alternative approach to the disinfection of water. However, most of the processes available today were developed for low pressure environments, which are plagued by the need for complex and expensive vacuum systems, batch-type processing, and difficult robotics handling. These characteristics make conventional plasma technologies economically viable only for applications where the economies of scale processing are targeted toward the creation of high value-added items.
Gas phase discharges have been studied extensively for their ability to sterilize microorganism-contaminated solid surfaces. However, technologies for decontaminating fluids, and water in particular, are considerably less developed. The destruction of living cells, such as Saccharomyces cerevisiae (yeast cells) and Bacillus natto, has been studied in pulsed high voltage cylindrical discharge reactors in various electrode configurations. These studies show that yeast cell populations in deionized water can be destroyed using a wire-cylinder electrode configuration under 20 kV/cm, 140 xcexcs pulse width, and 250 Hz pulse frequency conditions.
The pulsed high-voltage discharge-mediated formation of chemical species and their effects on microorganisms has also been studied. Using a needle-plate electrode configuration, the formation of .OH and .H free radicals has been monitored by Optical Emission Spectroscopy. The studies indicated that .OH and .H free radicals generated in situ by a discharge were not effective at killing yeast cells, although the H2O2 generated by the discharge added ex situ to a contaminated sample could be used to kill the cells.
Unfortunately, these pulse discharge experiments for decontaminating water employed a high voltage, pulsed discharge which generated filamentary non-stationary discharge channels, resulting in reactions having a very localized character, which tends to limit the effectiveness of the reactions for inactivating microorganisms.
Another approach to the disinfection of microorganism-contaminated water employs antimicrobial nanoparticles. Nanoparticles are important components in the development of catalytic, sensor, aerosol, filter, biomedical, magnetic, dielectric, optical, electronic, structural, ceramic and metallurgical applications. Nanoscale metallic particles exhibit volume and surface effects which are absent in the same material with dimensions in the micron range (i.e., 0.1 micron less than particle diameter less than 1 micron).
The use of colloidal suspensions of silver as antimicrobial agents is well known. Such use is resuming increased importance as antibiotic resistant bacteria become more prolific. Minimizing the silver particle sizes is believed to be important both from the stability of the colloidal suspension and for the efficacy against microbes.
Various processes to produce nanoparticles are known in the prior art. For example, U.S. Pat. No. 5,543,133, issued to Swanson et al., discloses a process of preparing nanoparticulate agents comprising the steps of: (i) preparing a premix of the agent and a surface modifier; and, (ii) subjecting the premix to mechanical means to reduce the particle size of the agent, the mechanical means producing shear, impact, cavitation and attrition.
Likewise, U.S. Pat. No. 5,585,020, issued to Becker et al., teaches a process of producing nanoparticles having a narrow size distribution by exposing microparticles to an energy beam such as a beam of laser light, above the ablation threshold of the microparticles.
Also, U.S. Pat. No. 5,879,750, issued to Higgins et al., teaches a process for producing inorganic nanoparticles by precipitating the inorganic nanoparticles by a precipitating agent for a microemulsion with a continuous and a non-continuous phase and concentrating the precipitated nanoparticles employing an ultrafiltration membrane.
Additionally, U.S. Pat. No. 6,540,495, issued to Markowicz et al., teaches a process for making a powder containing metallic particles comprising the steps of: (i) forming a dispersion of surfactant vesicles in the presence of catalytic metal ions; (ii) adjusting the pH to between 5.0 and 7.0; (iii) mixing the dispersion with a bath containing second metal ions; and; and, (iv) incubating the mixed dispersion at a temperature sufficient to reduce the second metal ions to metal particles having an average diameter between 1 to 100 nm.
CS Pro Systems advertises a high voltage AC processor producing nanoparticles of colloidal silver. The HVAC process is claimed to produce particle sizes between 0.002 to 0.007-9 microns by imposing an AC potential of 10,000 volts across two silver electrodes in a distilled water medium.
The production of large quantities of colloidal silver solutions required for industrial applications, such as water treatment or treatment of biological fluids, are not economical by using the electrolytic approach.
The prior art methods do not provide simple, convenient, low-cost methods for disinfecting water, and other dense media, contaminated with undesirable microorganisms.
One aspect of the invention provides a method for disinfecting a dense fluid medium, such as water, containing at least one undesirable microorganism. The method uses multiple spark discharges to inactivate the microorganisms in an intensely stirred liquid medium. The method comprises the steps of: providing a reaction vessel for containing a dense fluid medium containing at least one microorganism; charging the dense fluid medium into the reaction vessel; providing a first electrode comprising a first conductive material, the first electrode immersed within the dense fluid medium; providing a second electrode comprising a second conductive material, the second electrode immersed within the dense fluid medium and disposed opposite the first electrode; stirring the dense fluid medium between the first and second electrodes; applying an electric potential between the first electrode and the second electrode to create a discharge zone comprising a plurality of discharges to produce reactive species in the dense fluid medium; and exposing the microorganisms in the dense fluid medium to the reactive species in the dense fluid medium for a time sufficient to at least partially inactivate the microorganisms. The reactive species include electrons, ions, free radicals, and mixtures thereof which are capable of interacting with the microorganism to promote the inactivation of the microorganism. In a preferred embodiment, the first electrode is a rotating electrode and the second electrode is a static electrode. In this embodiment the dense fluid medium is stirred by the rotating motion of the first electrode.
Another aspect of the invention provides a method for disinfecting a dense fluid medium containing at least one microorganism using antimicrobial colloidal nanoparticles generated in a dense medium plasma (DMP) environment through multiple spark discharges in an intensely stirred liquid medium. The steps in this method are substantially the same as those described above, however, in this aspect of the invention at least one of the first conductive material or the second conductive material comprise a material having antimicrobial properties and the electric potential between the first electrode and the second electrode is sufficient to dislodge or dislocate antimicrobial nanoparticles from that material. A particularly preferred material having antimicrobial properties is silver.
Yet another aspect of the invention provides a two-step method for disinfecting a dense fluid medium containing at least one undesirable microorganism. In the first step of the two step method the dense fluid medium containing the at least one microorganism is exposed to reactive species created by multiple spark discharges in an intensely stirred medium. The reactive species are allowed to react with the microorganism to at least partially inactive the microorganism. The method for carrying out this first step has been described above. Briefly, a dense fluid medium containing at least one microorganism is disposed between two electrodes. The medium is stirred between the electrodes and an electric field sufficient to produce multiple spark discharges is applied between the electrodes to produce reactive species that interact with the at least one microorganism to promote its inactivation. In the second step of the two step method, the dense fluid medium containing the at least one microorganism is exposed to antimicrobial colloidal nanoparticles.
The second step of the process may be accomplished by mechanically mixing a solution containing antimicrobial nanoparticles, which may be a colloidal suspension, into the dense fluid medium. Such a solution may be produced by conventional means well known in the art or may be produced using a dense medium plasma reactor, as described in greater detail below. The mixing may take place before, during, or after the dense fluid medium has been exposed to the reactive species created by the multiple spark discharges in the first step of the process. Alternatively, the antimicrobial nanoparticles can be formed within the dense fluid medium by exposing the dense fluid medium to multiple spark discharges between a first and a second electrode, at least one of which is comprised of a material having antimicrobial properties.
In the two-step method, the first step and the second step may take place simultaneously or in tandem. For example, where at least one of the first or second electrodes is made from a material having antimicrobial properties and the voltage between the first and the second electrodes is high enough to dislocate antimicrobial nanoparticles from that electrode, reactive species and antimicrobial nanoparticles will be formed simultaneously in the same reaction vessel. Alternatively, the two step process may be carried out in a dual-stage dense medium plasma reactor having separate reaction stages, or containers, housed within a single reaction vessel which may be connected in parallel or, preferably, in series, to facilitate continuous production of the colloidal dispersion.
When carried out in a dual-stage dense plasma reactor, the method includes the steps of: providing a first reaction container for containing a dense fluid medium containing at least one microorganism; charging the dense fluid medium into the first reaction container, providing a first electrode comprising a first conductive material, the first electrode immersed within the dense fluid medium and housed within the first reaction container; providing a second electrode comprising a second conductive material, the second electrode immersed within the dense fluid medium and disposed opposite the first electrode within the first reaction container; applying an electric potential between the first electrode and the second electrode to create a discharge zone comprising a plurality of discharges, wherein the electric potential between the first and the second electrodes is high enough to produce reactive species in the dense fluid medium; providing a second reaction container for containing a dense fluid medium containing at least one microorganism the second reaction container connected to and in fluid communication with the first reaction container through an inlet port; charging the dense fluid medium between the first reaction container and the second reaction container; providing a third electrode comprising a third conductive material, the third electrode immersed within the dense fluid medium and housed within the second reaction container; providing a fourth electrode comprising a fourth conductive material, the fourth electrode immersed within the dense fluid medium and disposed opposite the third electrode within the second reaction container; applying an electric potential between the third electrode and the fourth electrode to create a discharge zone, wherein at least one of the third conductive material or the fourth conductive material comprises a material having antimicrobial properties; and further wherein the electric potential between the third and the fourth electrodes is high enough to dislocate antimicrobial nanoparticles from the electrode comprising the material having antimicrobial properties.
It should be understood that in the two-step process described above, the flow of the dense fluid medium may be from the first container (i.e. the container wherein reactive species are created) to the second container (i.e. the container wherein the antimicrobial species are created) or vice versa. Thus the phrase xe2x80x9ccharging the dense fluid medium between the first reaction container and the second reaction containerxe2x80x9d does not limit the flow of the dense fluid medium to one direction or the other, but merely indicates that the fluid is moving or circulating between the containers.
Still another aspect of the invention provides a method for producing a colloidal dispersion of nanoparticles of at least one conductive material in a dense fluid medium. The production of nanoparticles, and in particular antimicrobial nanoparticles, in this manner is well-suited for use with applications for disinfecting water, and other dense fluid media, contaminated with microorganisms. The method is based on the operation of a modified dense medium plasma reactor, which allows the initiation of multiple spark discharges in a very intensely stirred liquid medium. The method comprises the steps of: providing a reaction vessel for containing the dense fluid medium; charging the dense fluid medium into the reaction vessel; providing a first electrode comprising a first conductive material, the first electrode immersed within the dense fluid medium; providing a second electrode comprising a second conductive material, the second electrode immersed within the dense fluid medium and being near to the first electrode; stirring the dense fluid medium between the first and second electrodes; and imposing an electric potential between the first electrode and the second electrode to create a discharge zone, the electric field between the electrodes being sufficiently high to dislocate nanoparticles of at least one of the first conductive material or second conductive material from the respective electrode. Preferably, the electrodes are easily interchanged to facilitate changeover between dispersions. In a preferred embodiment, the first electrode is a rotating electrode and the second electrode is a static electrode. In this embodiment the dense fluid medium is stirred by the rotating motion of the first electrode.
An exemplary dense phase plasma discharge apparatus suitable for use with the invention may include a chamber forming a reaction vessel for the dense medium. A first electrode is mounted for a rotation about an axis in the chamber and has an end piece which is formed of conductive material with a planar surface. A plurality of pins are mounted in an array projecting from the planar surface. A second electrode is mounted in the chamber and has an end piece of conductive material with a planar surface, with the planar surfaces of the end pieces of the first and second electrodes separated from each other by a gap. The end pieces of the first and second electrodes, including the pins on the one end piece, may be formed of silver for efficiently producing colloidal silver. A motor may be coupled to the first electrode to selectively drive the first electrode in rotation. A magnetic coupling system may be utilized to couple the drive from the motor to the rotating electrode. The pins in the electrode are preferably formed in a spiral array. Rapid rotation of the electrode with the pins therein creates vigorous mixing and cavitation of the dense medium, such as water, between the upper and lower electrodes to enhance the action of the discharges taking place between the electrodes and thereby enhance the production of nanoparticles dislodged from the electrodes from the discharge.
Utilization of the method of the invention with silver electrodes may be used to produce colloidal silver which is highly effective as a bactericide and can be used for controlling viruses, spores, and other undesirable microorganisms.
Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.